Laser glasses with high damage threshold and method of making such glasses

ABSTRACT

LASER GLASSES HAVING HIGH RESISTANCE TO SELF-DAMAGE DURING OPERATION ARE FORMED IN ALL CERAMIC MELTING UNITS, IN THE PRESENCE OF A FINING AGENT TO ELIMINATE THE FORMATION OF METALLIC INCLUSIONS AND SEMI-CONDUCTIVE INCLUSIONS AND USING BASE GLASS COMPOSITIONS WITH REDUCED TENDENCY TOWARD MICROPHASE SEPARATION AND DEVITTRIFICATION. THE MAJOR INGREDIENTS OF THE GLASS BATCHES INCLUDE SILICON DIOXIDE, ALKALI AND ALKALINE EARTH NITRATES, CARBONATES AND FLUORIDES. THE METHOD INVOLVES INTRODUCING OXIDIZING COMPOUNDS INTO THE REACTION MIXTURE IN A SUFFICIENT AMOUNT THAT DURING THE REACTION AND FINING PHASE, OXIDIZING CONDITIONS ARE MAINTAINED THROUGHOUT THE ENTIRE VOLUME OF THE GLASS MELT.

United States Patent ,749 LASER GLASSES WITH HIGH DAMAGE THRESHOLD ANDMETHOD OF MAKING SUCH GLASSES Emil W. Deeg, Woodstock, Conn., and RobertE. Graf, S outhhridge Mass, assignors to American Optical Corporation,Southbridge, Mass. No Drawing. Continuation of abandoned applicationSer. No. 148,225, May 28, 1971, which is a continuation of abandonedapplication Ser. No. 801,800, Feb. 24, 1969. This application Apr. 20,1973, Ser. No. 353,144 The portion of the term of the patent subsequentto Feb. 13,1990, has been disclaimed Int. Cl. C03b 5 16'; C03c 3/00;C09k N04 US. Cl. 252301.4 F 14 Claims ABSTRACT OF THE DISCLOSURE Laserglasses having high resistance to self-damage during operation areformed in all ceramic melting units, in the presence of a fining agentto eliminate the formation of "metallic inclusions and semi-conductiveinclusions and using base glass compositions with reduced tendencytoward microphase separation and devitrification. The major ingredientsof the glass batches include silicon dioxide, alkali andalkaline earthnitrates, carbonates and fluorides. The method involves introducingoxidizing compounds into *the reaction mixture in a sufiicient amountthat during' the reaction and fining phase, oxidizing conditions aremaintained throughout the entire volume'of the glass melt.

This is a continuationof application Ser. No. 148,225, filed May 28,197.1 nowabandoned, which was a continuation of Ser. No. 801,800, filedFeb. 24, 1969, now ab n Qnt-d.

When in operation, lasers produce tremendous amounts of -energy,--andthis energy internally of the glass of the laser causes metallicandsemi-conductive inclusions in the glass to-expand, move, vaporize,'etc., and thus crack or explode. the laser glass. The greatest amountof energy density-that can be propagated without damage to the glass istermed the damage threshold of that glass. Heretofore, laser glasses'have been prepared in either platinum or-ceramic 'pots. However, at thehigh temperature required-to melt the laser glass,-.platinu.m vaporizes,is oxidized in the gaseous state, decomposes on cooling, andprecipitates into the melted glass as platinum inclusions. In the knownprior art no attempt'has been made to consider, the: composition-of theglass itself and its contributions to the formation of metallic orsemi-conductive inclusions.-'Ihe prior, art seems vto be concerned onlywith the effects ofthemelting pots themselves or the atmosphere over theglass in the'pots. All known laser glasses include fining agents (bubbleformers), and the most widely used fining agent is antimonoy oxide, Sb O;A further problemwith laser glasses is an effect called solarization,which can occur when glasses are exposed to ultraviolet radiation. Ifthat process takes place preferably in small areas within the structureof a laser glass,- dark spots are formed which absorb the energyproducedby-the laser and act as damage centers.

selfidamageof glassxlaserrods during operation is usually causedbYxlOCtiliZd energy concentrations. at the metallic or semi-conductiveinclusions in the rodsor on the-surfacey,of-therods. As stated above, ithas been widely accepted knowledge in the prior art that the onlymetallic inclusions considered to be important in the selfdamage oflaser rods are the platinum or other metal inclusions originatnig fromcrucibles and/or stirrersfor the melt or even improperly protectedthermocouples and heating elements in the furnace. In order to avoidsuch inclusions, laser glasses for high power systems have Patented Aug.20, 1974 heretofore been manufactured in all ceramic melting units. Inother attempts to avoid the formation of the metal inclusions, themelting and/or refining of the laser glasses has been conducted under aneutral atmosphere. The prior art, furthermore, indicates that someglass melts performed under balanced redox conditions result in glasseswhich are more stable to solarization.

According to the present invention we have discovered that a laser glassmelt containing alkali metal or alkaline earth metal compounds such asnitrates, carbonates, sulfates, etc. or mixtures of such compounds underoxidizing conditions produces a high damage threshold laser glass. Themethod. of the invention includes selecting a base glass compositionwith a low tendency towards microphase separation and/or localizedcrystallization with combinations of batch compositions and melting suchcompositions in ceramic crucibles under oxidizing conditions. The baseglass composition includes ions having a high field strength which givesa laser glass with a high damage threshold. The method of the inventioneliminates the formation of metallic or semi-conductive inclusions inlaser glass melts. Further, there is radiations by providing a glasscomposition having a low eliminated internal breakdown of the glass dueto laser tendency to form localized areas containing mobile ions andsources of internal field emission which in turn is known as a reasonfor dielectric breakdown.

Preferably the batch of laser glass should not contain compounds of thenoble metals and/or any of the follow ing elements; antimony, arsenic,zinc, lead, tin, cadmium, bismuth, selenium, tellurium or copper, andgenerally transition elements, 4a, 5a and 6a group metals. The batch oflaser glass must contain fining agents which may be salts of alkali andalkaline earth metals, preferably nitrates of sodium, potassium andammonia. Nitrates are preferred since they promote an oxidizing system.The glass batch is thoroughly mixed eliminating the introduction ofmetallic particles and/ or reducing materials such as organic dustparticles, chips of plastics, silicon carbide particles, etc., and themelting and fining is done in an all ceramic unit. On completion offining and conditioning, the glass is shaped using known techniques suchas casting, extruding, pressing or drawing.

Included among the objects and advantages of the present invention is alaser glass with a high damage threshold with essentially no metallic orsemi-conductive inclusions.

Another object of the invention is to provide a method of making a laserglass having a high damage threshold and without the formation oflocalized metallic and/or semi-conductive inclusions.

A further object of the invention is to provide fining agents for laserglass which consists essentially of alkali, and alkaline metal, andammonium salts.

These and other objects and advantages of the invention may bereadilyascertained by referring to the following description andexamples wherein .the glass melt is made according to known techniquesand the resulting glass is shaped by such. known techniques casting. ex,truding, pressingor drawing. 1 1

In general, the composition of the laser glass, made according to theinvention, contains ingredients found in the following list, within theweigh t p'erc ent listed:

we I

Wt. percent SiO 40-80 Li o 0-20 Na O 042 K 0. 0-25 Rb O 0-10 Cs O 0-10TABLE a reduced likelihood of mic ophase separation and/or Wt'percentcrystallization. Additionally, these glasses are composed- Mgo -20 ofions having a high field strength and have a high g g damage threshold.'7,

Bao I: 045 The method of the invention includes add ng a su fiiclent 0amount of oxidizing composition of ammonia, alkali metal A1 0 2 oralkaline earth metal salts of a mineral acid, such as Nd 0 2-6 nitratesalts, to maintain an oxidizing condition through? Yb O 04 out thevolume of the glass melt, through the'finin'g'and conditioning stages.This oxidizes all metallic salts to the The Summauon of an the weightpercents must equal oxide state which are maintained asoxidesin thefinished 100 weight percent, and contains at least one alkali metal oralkaline earth oxide. glassv Specific examples of the ingredients forglass batches The method PP t0 the glassvmelis results l used in theprocess are shown in the following two tables: crease of the damagethreshold, and in one case 1t 1n- TABLE II v a 4 5 6 7 s 0 10 11' 12 1s14 Batch number 1 CaF 4MgCO -Mg(OH) ziHzO 102. 08 102. 08 SrCOs 73. 84

64.26 64. 26'. 6. 43 6. 4a B 8.40 8.40 Nda 12 60 21 00 16. 00 16. 0020.00 16.00 16. 00 (NHDNO'I The batch compositions shown in Columns 1,2, 9, 12 creased the threshold of the base glass mix from about and 13of Table II yield oxide compositions in weight 10 .l/cm. to over 45J/cm}. In another test the damage percent shown in Table III below andlabeled Columns 1, threshold was increased from about 10 J/cm. toover.3l 2, 9, 12 and 13 respectively to correspond to the same J/cm.based on tests made with pulses of one micro columns in Table II. secondduration.

The fining agents are selected from minerals and salts TABLE III ofammonia, alkali metal, alkaline earth metals, and rare earth metals andinclude such salts as NaCl, Na SO 1 2 9 12 13 NaHS, NH NO KBr, KI, CeOetc. The conventional i1 0, 04.04 62.02 52.58 51 52 tiggg fining agentsantimony oxide (Sb O and arsenic oxide (As O are not used since theymaybe readily reduced, at the temperatures necessary to melt glass, tothe metal forming localized metallic particles. 1 I

The laser glass of the invention is characterized by a high damagethreshold; having no metallic inclusions of either the noble metals orof such base metals as antimony and arsenic, the oxides of which arereadily reduced to the metallic state; without local semi-conductiveinclusions of such compounds as ZnS, PbS, ZnO, CdS, etc.; with a greatlyreduced tendency toward microphase separation and/or devitrificationwhereby there are no 10ml- The glasses formed fr th i di t are composedized areas formed having a higher concentration of mobile of ions havinghigh field strength producing a laser glass 10118 Such as the glass q fOut Occurwith a high damage threshold. These glasses, also, have ranceof localized internal field emlssion of electrons. low tendency to formlocalized areas containing mobile 55 We 6131111! 1 i i 1 1 ions andsources of internal field emission, which would A method of P fl g iglass having a hlgh lead to dielectric breakdown if greater. damagethreshold comprising rruxmg a major portion'of silicon dioxide and aminor amount of neodymium oxide 7 with a minor amount of at least one ofan inorganic com- TABLE IV pound of aluminum, boron, and ytterbium andwith a minor amount of a fining agent selected from alkali metal,alkaline earth metal and ammonium salts'of mineral acids, cerium oxide,fusing and conditioning said mix in 'a ceramic vessel in the presence ofa sufiicient' amount of an inorganic oxidizing'compound formed of anammonia,

alkali metal or alkaline earth metal with an oxidizing-inorganic acid tomaintain an oxidizing condition throughout the entire volume of theglass melt, and casting the resultant glass. i 1 1M0 7 2. A methodaccording to claim 1 wherein saidfining agent is NaCl. 1

i r 3; A method according to claim 1 wherein said-fining Glasscompositions formed'from ingredients in Tables agent is Na SO II and- IVhave a reduced possibility of formation of 4. A method according toclaim l'wherein sa'id fin'ing microphase separation and/ordevitrification so there is agent is NaHS. I P

5. A method according to claim 1 wherein said fining agent is CeO 6. Amethod according to claim 1 wherein said fining agent is an alkali metalhalide.

7. A method according to claim 1 wherein said fining agent is KBr.

8. A method according to claim 1 wherein said oxidizing compound is NHNO 9. A method according to claim 1 wherein said oxidizing compound isNaNO 10. A method according to claim 1 wherein said oxidizing compoundis KNO 11. A method for producing a laser glass having a high damagethreshold comprising:

mixing a major portion of silicon dioxide and a minor amount ofneodymium oxide with a minor amount of at least one of an inorganiccompound of aluminum, boron, or ytterbium and at least one fining agentselected from the group of alkali metals, alkaline earth metals andammonia compounds; melting and conditioning said mix in anon-contaminating vessel with a sufficient amount of at least one of aninorganic oxidizing compound of an alkali metal or alkaline earth metalto maintain an oxidizing condition throughout the entire volume of theglass melt; and forming resultant glass. 12. A method for producing alaser glass having a high damage threshold comprising:

mixing a major portion of silicon dioxide and a minor amount ofneodymium oxide with a minor amount of at least one of an inorganiccompound of aluminum, boron, or ytterbium and at least one fining agentselected from the group of alkali metals, alkaline earth metals andammonia compounds; melting and conditioning said mix in anoncontaminating vessel; maintaining an oxidizing condition throughoutthe entire volume of the glass melt during said melting andconditioning; and forming a resultant glass.

13. The method of claim 12 wherein the resultant glass consists of thefollowing metal oxide compositions within the weight percent ranges:

Weight percent provided that the total equals 100 weight percent, saidglass being without metal inclusions and semi-conductive inclusions andbeing free of reducing materials and having a reduced tendency towardmicrophase separation and devitrification.

14. A laser glass product characterized by a high damage threshold,without metallic and semi-conductive inclusions, being free of reducingmaterials and having a reduced tendency toward microph-ase separationand devitrification made in accordance with the method of Claim 12.

References Cited UNITED STATES PATENTS 3,716,349 Deeg et al. -134X3,471,409 10/1969 Lee, Jr. et al. 65-134X 3,528,927 9/1970 Graf et al.252301.4F 3,471,408 10/ 1969 Young 252301.4 F 3,640,890 2/1972 Lee et al25230l.4 F

S. LEON BASHORE, Primary Examiner F. W. MIGA, Assistant Examiner U.S.Cl. X.R.

